1. Field of the Invention
This invention relates to a sintered permanent magnet of an R-Fe-B system, that is, containing R (R is a rare earth element inclusive of Y throughout the disclosure), Fe and B.
2. Prior Art
Typical of high performance rare earth magnets are powder metallurgical Sm-Co base magnets having an energy product of the order of 32 MGOe which have been commercially produced in a mass scale. These magnets, however, undesirably use expensive raw materials Sm and Co. Among the rare earth elements, those elements having a relatively low atomic weight, for example, Ce, Pr and Nd are available in plenty and less expensive compared to Sm. Further Fe is less expensive than Co.
From these aspects, R-Fe-B system magnets such as Nd-Fe-B magnets were recently developed as seen from Japanese Patent Application Kokai No. 46008/1984 disclosing sintered magnets and Japanese Patent Application Kokai No. 9852/1985 disclosing rapidly quenched ones.
For sintered magnets, the powder metallurgical process of the conventional Sm-Co system (melting.fwdarw.casting.fwdarw.ingot crushing.fwdarw.fine milling.fwdarw.compacting.fwdarw.sintering.fwdarw.magnet) is applicable and high magnet performance is expectable. The R-Fe-B magnets, however, are less heat stable than the Sm-Co magnets as demonstrated by a differential coercivity .DELTA.iHc/.DELTA.T as great as -0.60 to -0.55%/.degree.C. in the range of from room temperature to 180.degree. C., and a significant, irreversible demagnetization upon exposure to elevated temperatures. Therefore, the R-Fe-B magnets are rather impractical when it is desired to apply them to equipment intended for high temperature environment service, for example, electric and electronic devices in automobiles.
For reducing the irreversible demagnetization of R-Fe-B magnets by heating, Japanese Patent Application Kokai No. 165305/1987 proposes to substitute Dy for part of Nd and Co for part of Fe.
Although Dy substitution can improve coercive force iHc at room temperature and Co substitution can increase iHc and improve .DELTA.Br/.DELTA.T to some extent, the inventors have found that it is impossible to achieve a substantial reduction of .DELTA.iHc/.DELTA.T by merely adding Dy and Co. As shown in the above-cited patent publication, samples having larger amounts of Dy substituted have relatively low percent irreversible demagnetization, but at the sacrifice of maximum energy product (BH) max.